Al'Bina I. Orlova

The microstructure and thermophysical properties (specific heat capacity, thermal diffusivity, thermal conductivity) of fine-grained ceramic composites based on yttrium-aluminum garnet Y_2.5Nd_0.5Al₅O₁₂ (YAG:Nd) with different molybdenum contents (10, 20 and 40 vol %) were studied. Submicron garnet powders of Y_2.5Nd_0.5Al₅O₁₂ were obtained by the coprecipitation method; YAG:Nd + Mo powder compositions with the YAG:Nd core–Mo shell structure were obtained by deposition of molybdenum onto the surface of garnet particles; samples of ceramic composites were obtained by the method of spark plasma sintering (SPS). Electron microscopy and X-ray phase analysis were used to study the microstructure and phase composition of the composites. YAG:Nd + Mo composites have a high relative density (98.1–99%) and a uniform fine-grained microstructure with a garnet grain size of 2–3 μm. Sintered YAG:Nd + Mo composites at room and elevated temperatures (up to 1100°C) have a high thermal conductivity coefficient exceeding the thermal conductivity coefficient of uranium dioxide UO₂, which allows using these materials as heat-resistant inert fuel matrices. It was shown that higher thermal conductivity of composites is ensured at a content of at least 20 vol % Mo. In composites with the addition of 20 and 40% Mo, the thermal conductivity coefficient at 1100°C reaches 7.0 and 8.8 W m^–1 K^–1, respectively.

The chemical stability of Nd_0.33Zr₂(PO₄)₃ fine-grained ceramics, which can be used for immobilization of rare earth elements (REE) that are part of high-level waste was studied. Single-phase Nd_0.33Zr₂(PO₄)₃ submicron powders with the structure of the mineral kosnarite were prepared by colloid-chemical synthesis. Powders were prepared by successive annealing at 600, 800, and 900°C for 6 h at each stage. Nd_0.33Zr₂(PO₄)₃ ceramic was Nd_0.33Zr₂(PO₄)₃ ceramics was produced using spark plasma sintering method (SPS). The relative density of the ceramic was 89.9%; the average grain size was 5–20 μm. The chemical stability of the ceramic in the static mode at 90°C in distilled water and mineral water, as well as in acidic media (0.1 M HCl) and alkaline media (0.01 M NaOH), was studied. The ceramics have high hydrolytic stability. The influence of the contact medium on the rate and mechanism of Nd leaching from the surface of Nd_0.33Zr₂(PO₄)₃ fine-grained ceramic samples was studied. The de Groot-van der Sloot model was used to analyze the resulting leaching rate R_i against time t of experiment. Nd leaching occurs by dissociation of Nd from the ceramic surface in acidic medium, diffusion from the inner layers in alkaline medium and mineral water, and dissolution of the ceramic surface layer in distilled water.

Oxide Y2.5Nd0.5Al5O12 (YAG:Nd) with garnet structure was synthesized in the powder and ceramics forms. Fine-grained YAG:Nd ceramics with a relative density of ~99% were obtained by the Spark Plasma Sintering method (SPS). The radiation resistance of ceramics was studied under irradiation with swift Xe-ions (E = 146 MeV). A gradient defect structure is formed in irradiated ceramics, varying from layer to layer. The strained YAG phase formed as a result of Xe ions irradiation is localized in a near-surface layer with a thickness of ~5 μm. Full amorphization of the samples was observed under irradiation with a fluence of 1x10^13 cm-2. The calculated critical fluence was 6.5x10^12 cm-2, which corresponded to 0.03 dpa. The microhardness of the surface layers of irradiated ceramics is less than the central layers, and, in general, decreases with increasing ion fluence.

Submicron-grade powders of Na_1-xZr₂(PO₄)_3-x(XO₄)_x compounds (hereafter referred to as NZP) and Ca_1-xZr₂(PO₄)_3-x(XO₄)_x compounds (hereafter, CZP), X = Mo, W (0 ≤ x ≤ 0.5) were obtained by sol-gel synthesis. The compounds obtained were studied by X-ray diffraction phase analysis and electron microscopy. An increase in the W or Mo contents was shown to result in an increase in the unit cell volume of the NZP and CZP crystal lattices and in a decrease in the coherent scattering region sizes. Thermal expansion behavior at high temperatures of synthesized NZP and CZP compounds has been investigated. The dependencies of the parameters a and c on the heating temperature, as well as the temperature dependence of the crystal lattice unit cell volume V in the range from the room temperature up to 800 °C, were obtained. The dependencies of the average thermal expansion coefficient (α_av) and of the volume coefficient (β) on the W and Mo contents in the compositions of NZP and CZP compounds were studied. Ceramics Na_1-xZr₂(PO₄)_3-x(XO₄)_x with relatively high density (more than 97.5%) were produced by spark plasma sintering (SPS). The increase in the W or Mo contents in the ceramics leads to an increase in the relative density of NZP and to a decrease of the optimum sintering temperature. The mean grain size in the NZP ceramics decreases with increasing W or Mo contents. The study of strength characteristics has revealed that the hardness of the NZP ceramics is greater than 5 GPa, and that the minimum fracture toughness factor was 1 MPa·m^1/2.

This paper introduces the results of hydrolytic stability tests and radiation resistance tests of phosphate molybdates and phosphate tungstates Na_1−xZr₂(PO₄)_3−x(XO₄)_x, X = Mo, W (0 ≤ x ≤ 0.5). The ceramics characterized by relatively high density (more than 97.5%) were produced by spark plasma sintering (SPS) of submicron powders obtained by sol–gel synthesis. The study focused on hydrolytic resistance of the ceramics in static mode at room temperature. After 28 days of testing in distilled water, the normalized leaching rate was determined. It was found that the ceramics demonstrated high hydrolytic resistance in static mode: the normalized leaching rates for Mo- and W-containing ceramics were 31·10⁻⁶ and 3.36·10⁻⁶ g·cm⁻²·day⁻¹, respectively. The ceramics demonstrated high resistance to irradiation with 167 MeV Xe⁺²⁶ multiple-charged ions at fluences ranging from 1·10¹² to 6·10¹³ cm⁻². The Mo-containing Na_0.5Zr₂(PO₄)_2.5(XO₄)_0.5 ceramics were shown to have higher radiation resistance than phosphate tungstates. Radiation was shown to trigger an increase in leaching rates for W and Mo in the crystal structure of NZP ceramics.

This paper reports on the preparation of xenotime-structured ceramics using the Spark Plasma Sintering (SPS) method. Y_0.95Gd_0.05PO₄ (YPO₄:Gd) phosphates were obtained using the sol-gel method. The synthesized powders were nanodispersed and were agglomerated (the agglomerates sizes were 10–50 µm). The ceramics had a fine-grained microstructure and a high relative density (98.67 ± 0.18%). The total time of the SPS process was approximately 18 min. The sintered high-density YPO₄:Gd ceramics with a xenotime structure were irradiated with ¹³²Xe⁺²⁶ ions with 167 MeV of energy and fluences in the range of 1 × 10¹²–3 × 10¹³ cm⁻². Complete amorphization was not achieved even at the maximum fluence. The calculated value of the critical fluence was (9.2 ± 0.1) × 10¹⁴ cm⁻². According to the results of grazing incidence X-ray diffraction (GIXRD), the volume fraction of the amorphous structure increased from 20 to 70% with increasing fluence from 1 × 10¹² up to 3 × 10¹³ cm⁻². The intensity of the 200 YPO₄:Gd XRD peak reached ~80% of the initial intensity after recovery annealing (700 °C, 18 h).

The hydrolytic stability of ceramics based on Y_2.5Nd_0.5Al₅O₁₂ oxide with a garnet structure obtained by the spark plasma sintering (SPS) method has been studied. The tests were carried out in distilled water under hydrothermal conditions in an autoclave and, for comparison, in a static mode at room temperature. The mechanism of leaching of Y and Nd from the ceramics was investigated. It has been shown that at “low” temperatures (25 and 100 °C), the destruction of pores occured, and the intensity of the leaching process was limited by the diffusion of ions from the inner part of the sample to the surface. At “high” test temperatures (200 and 300 °C), intense destruction of the ceramic grain boundaries was observed. It was assumed that the accelerated leaching of neodymium is due to the formation of grain-boundary segregations of Nd³⁺ in sintered ceramics.

Crystalline ceramics are intensively investigated as effective materials in various nuclear energy applications, such as inert matrix and accident tolerant fuels and nuclear waste immobilization. This paper presents an analysis of the current status of work in this field of material sciences. We have considered inorganic materials characterized by different structures, including simple oxides with fluorite structure, complex oxides (pyrochlore, murataite, zirconolite, perovskite, hollandite, garnet, crichtonite, freudenbergite, and P-pollucite), simple silicates (zircon/thorite/coffinite, titanite (sphen), britholite), framework silicates (zeolite, pollucite, nepheline /leucite, sodalite, cancrinite, micas structures), phosphates (monazite, xenotime, apatite, kosnarite (NZP), langbeinite, thorium phosphate diphosphate, struvite, meta-ankoleite), and aluminates with a magnetoplumbite structure. These materials can contain in their composition various cations in different combinations and ratios: Li–Cs, Tl, Ag, Be–Ba, Pb, Mn, Co, Ni, Cu, Cd, B, Al, Fe, Ga, Sc, Cr, V, Sb, Nb, Ta, La, Ce, rare-earth elements (REEs), Si, Ti, Zr, Hf, Sn, Bi, Nb, Th, U, Np, Pu, Am and Cm. They can be prepared in the form of powders, including nano-powders, as well as in form of monolith (bulk) ceramics. To produce ceramics, cold pressing and sintering (frittage), hot pressing, hot isostatic pressing and spark plasma sintering (SPS) can be used. The SPS method is now considered as one of most promising in applications with actual radioactive substances, enabling a densification of up to 98–99.9% to be achieved in a few minutes. Characteristics of the structures obtained (e.g., syngony, unit cell parameters, drawings) are described based upon an analysis of 462 publications.